Solid domestic waste collected in the United States at the present time, amounts to more than 200,000,000 tons annually and of this amount of solid waste only 27% is amenable to recycling. The remainder must be disposed of in some fashion and for that purpose disposal in landfills, incineration and the like are among the most common methods. Landfills contaminate the environment, are unsightly and must cover large areas when such large volumes of waste are considered. They have been located near residential areas in the past because of the expense of transportation and increasingly there is pressure to avoid siting landfills near large residential areas and hence a search with increased importance for other ways of dealing with such solid wastes.
Pyrolytic decomposition is one method of disposing of solid waste which has the advantage over the use of landfills, in that valuable product may be obtained from the pyrolysis or incineration process.
This method need not involve preliminary sorting and in the past was carried out at about 3000.degree. F. to produce steam and a synthesis gas in the form of a mixture of carbon monoxide and hydrogen (see U.S. Pat. No. 2,134,944 and Martin, K. and Bastock, T. W., "Waste Minimization: A Chemist's Approach" Royal Society of Chemistry, 1994, page 111). However, in such systems the products obtained require complex purification. The process requires the use of expensive construction material because of the high temperatures used, involve a high consumption of fuel, are complex and are complicated by the presence of chlorine-containing substances in the refuse.
A pyrolysis at about 1500.degree. F. is known. This temperature is required for complete destruction of low temperature resins (Kaiser, E. R. and Friedman, S. B. "The Pyrolysis of Refuse Components, Incinerator and Solid Waste Technology" the American Society of Mechanical Engineers, 1975, pages 247-256). A similar method has been proposed to obtain a liquid fuel from refuse (see "The Star Ledger", Jan. 29, 1998). Problems were involved with this approach as well because of the presence of chlorine and organic compounds thereof, especially polyvinylchloride. In general the presence of chlorine as an aggressive and corrosive substance in solid wastes has proved to be problematical.
In the past it has been found that incinerators operate with a heat recovery coefficient of 65% or less (Brown, A., Evermy P. and Ferrero, G. L. "Energy Recovery through Waste Combustion", Elsevier Applied Science, Inc., 1988, page 252). In such systems it proved to be impossible to neutralize discharged hydrogen chloride during the combustion because of a difficulty in mixing neutralizing agents into the system. When neutralization can be carried out, a product, for example, a chloride salt in the form of molten calcium chloride can be produced which can block the interstices and prevent penetration of air into the system and hence efficient combustion (Brunner, Calvin R. "Handbook of Incineration Systems", McGraw Hill Inc., 1988, page 12.6). Large amounts of fuel are required, for example 256.9 liters per ton of refuse at furnace temperatures of about 2200.degree. F. (page 24.6).
The literature apparently recognizes that the use of solid neutralizing agents is possible only in a fluidized bed combuster. It has been found that fluidized bed systems require substantial amounts of energy to provide the suspending blast of fuel, amounting say to 20.5 to 23.5 cubic meters of fuel or suspending gas per ton of refuse processed (see DeKadt, M. and Saphire, D., "Burning Garbage in the U.S. Practice vs. State of the Art" Inform Inc., 1991 pages 58 and 132).
In addition, incineration in fluidized bed combusters requires the use of refuse particles of a uniform size (usually 1" to 1.5"), the absence of glass and easily melted metals, such as aluminum and like restrictions: (Brunner, Clavin R. "Handbook of Incineration Systems", McGraw Hill Inc., 1988, pages 13.16-13.18).
U.S. Pat. Nos. 4,800,824 and 5,445,087 describe two such processing of refuse with preliminary pyrolysis followed by combustion of pyrolysis-generated gases. These processes can give rise to caking of refuse on the walls or surfaces of the pyrolysis chamber, restrictions in the handling of polyvinylchloride because of the formation of aggressive and corrosive HCL and portions of the environment with salts of heavy metal in addition to other drawbacks.
A continuous incineration method as described in U.S. Pat. No. 5,289,787 permits reburning of pyrolytic residues but has the disadvantages already mentioned.
U.S. Pat. Nos. 4,353,713, 4,448,558 and 4,597,771 disclose the processing of coal together with a light fraction of municipal solid waste in which there is a preliminary sorting and crushing of such waste, a mixture of the waste with limestone, a pyrolysis of the resulting mixture with purification of the pyrolysis gases and extrusion of certain products, a combustion of the residue and use of liberated heat and the heat of an exothermic reaction between the limestone or dolomite and carbon dioxide. These processes, however, can lead to pollution of the environment with toxic salts of heavy metals and are unable to process solid plastic wastes in spite of the fact that limestone and dolomite may be present to neutralize HCL released by decomposition of the plastics. These drawbacks of the earlier systems can be explained by the fact that calcium chloride which tends to be formed is in a molten state under the high conditions of these processes and causes waste to stick together and lump up and reduces the permeability of the waste to gas flow. The molten CaCl.sub.2 also interferes with the ability of components of the pyrolyzable mass to intermix. The melting point of calcium chloride is 1358 to 1376.degree. F., well below 2000 to 2400.degree. F. of the system and within the range of 1200 to 2400.degree. F. of the system and within the range of 1200 to 1800.degree. F. maintained in the gasification vessel. The molten calcium chloride, therefore, makes the processing of polyvinylchloride and other chlorine-containing polymers practically impossible.
High temperatures in combustion chambers of say 1500 to 1800.degree. F. means that salts of heavy metal (usually chlorides or sulfates) are present in a molten state since the melting point of these salts generally lies between 467.degree. F. and 1678.degree. F. Vapor of these salts are also present and are in equilibrium with the molten salts. Because the partial pressures of these vapors are low, by and large they are entrained in the flue gas away from the combustion chamber and can enter the ambient atmosphere. The low concentrations prevent them from being separated out although they are of levels which can pose substantial problems.
Finally it should be noted that in practically all of these processes, there is the danger of formation of secondary contaminants such as chlorobenzenes, chlorophenols, PCBs, dioxins and the like. Heavy metal salts are detectable in the incinerator atmosphere as well as in the ash and slags (see "Earth in the Balance, Ecology and the Human Spirit", Houghton Mifflin Co. 1993, page 156).
Mention can also be made of the plasma techniques for disposing of solid wastes and which produce fuel gases and do not form the secondary contaminants mentioned above. However, these processes cannot eliminate heavy metals and their salts and the product flue gases which the processes produce have substantially lower heat content than gases produced by other techniques.